One type of data interface uses changes in the magnitude of a current to transmit data signals. In the simplest case, one state is represented by the presence of a current flow and another by the absence of current. An output circuit of this type using a switch S to connect a current driver through the output nodes to a load R.sub.L is shown in FIG. 1a.
Other conventional data interfaces, such as those used in SCSI architecture, require a current to be always present on an active interface. The direction of current flow indicates whether the transmitted signal is a logic 0 or a logic 1. A conventional current steering output buffer of this type is illustrated in FIG. 1b. The circuit contains a current driver which is connected to the output nodes by a current switch. In a first state, switch pair S.sub.A is closed, switch pair S.sub.B is open, and the current driver contributes a current I to the total load current. In a second state, switch pair S.sub.B is closed, switch pair S.sub.A is open, and the current driver contributes a current of -I to the total load current. In conventional systems, current may be turned off by opening both switch pairs S.sub.A and S.sub.B.
As performance of integrated circuits continues to increase, the limited number of I/O (input/output) pins demands better utilization. Incorporating an output signal with more than the two conventional digital output states over a wire allows the interconnect of the I/O to carry a larger data bandwidth. Previous attempts to do this have used signalling via multiple voltage levels. However, in many circumstances, it is preferable to use current signaling instead. Further, conventional circuits generate switching noise at either of the power supply leads as the amount of power flowing into the output circuit is continuously changed to thereby vary the output signal level.